In a multi-node Radio Local Area Network (RLAN) communication system it is often necessary to detect the presence of other radio services that may be sharing the same radio band. The RLAN system may need to avoid use of the radio channels that the other radio services are using.
Such a situation occurs in practice for RLAN systems operating in 5 GHz bands (e.g. 5.250-5.350 GHz and 5.470-5.725 GHz) that are shared with primary radio determination services such as RADAR systems. In this case the RLAN system is required to monitor and detect the RADAR signals and to select alternate channels when RADAR signals are detected. Generally, the interfering signals are difficult for the RLAN devices to distinguish from other in-band noise. The RLAN system must be careful to distinguish between RADAR signals and other sources of interference before electing to change its operation to a new channel as frequent reselection of a new channel by the RLAN will cause degradation in its performance.
Existing RLAN equipment makes use of an RLAN receiver to detect potentially interfering signals when the device is not transmitting. These signals are then filtered by matching their pulse width and inter-pulse timing against the defined patterns for the RADAR signals. If a pattern match is found (i.e. matching the pulse duration, pulse count, and pulse timing interval), the RLAN device abandons the channel it is currently using and looks for another channel that is free of RADAR signals. The regulations require that each new channel be scanned for at least a minute to determine that it is not being used by RADAR systems. This means that there is at least an interruption of a minute before the RLAN device can resume service to its subtending mobile nodes (i.e. a customer's laptop PCs). In the most common deployments of individual RLAN access points this interruption, while undesirable, may not be too objectionable. For applications such as Web-browsing, the interruption may be unnoticeable if the customer is reading a page and not downloading new material. Even if there is downloading during the interruption, the delay may be masked by the server response delays.
The IEEE sub-committee 802.11h has extended the 801.11 RLAN radio standard protocol to include messages and actions to permit the above solution for single access point deployments and for control of subtending mobile nodes.
For configurations such as a multi-node mesh network in which the RLAN channels are being used to carry concentrated traffic across a network from multiple nodes, any interruption, especially one as long as a minute, is a major disruption that will affect service to multiple customers. In addition, because of the heavy use of the RLAN channels in the multi-node network, there is increased intra-system noise and hence an increased possibility that the RLAN devices will falsely detect a RADAR signature and change channels unnecessarily, with consequent degradation of service. Only a very minimum interruption can be tolerated if, for example, the RLAN system is being used for high quality services such as speech (e.g. Voice over IP (VoIP)) or video streaming.
In some systems, the channels that are vacated due to the detection of a RADAR signal cannot be re-occupied by the RLAN system for at least half an hour after the last RADAR signal on the channel is detected. Thus, false detection and unnecessary channel re-assignments will quickly exhaust the supply of available channels. There are about 12 channels in the 5.470-5.725 GHz band.